This invention relates in general to clocks that can synchronize to the time of a precise time standard, such as by decoding a low frequency radio signal that contains precise time information. More particularly, a preferred embodiment of the invention relates to clocks of the radio-controlled type that incorporate an improved display for viewing under all typical lighting conditions, that utilize a separate power transformer to reduce electromagnetic interference with a received low frequency radio signal, that provide a visual indication of the status of the received radio signal, and that provide an indication of the relative strength of the received low frequency time signal so that the user can reposition the radio-controlled clock for the best signal reception.
Radio-controlled clocks (RCCs) are known in the prior art, and have been commercially available for some time. RCCs receive and decode a low frequency radio signal that is coded with precise time information. Such clocks are sometimes called “atomic” clocks. However, “atomic” is a misnomer for the radio-controlled clocks that are commercially available to the public because atomic clocks utilize an internal atomic oscillator to keep highly accurate time.
However, the National Institute of Standards and Technology (NIST) operates and maintains a highly accurate atomic clock in Boulder, Colo., USA. The NIST also operates a radio station under the call letters WWVB near Fort Collins, Colo. Radio station WWVB transmits a low frequency radio signal that contains precise time information. The frequency of the radio signal is 60 KHz, well below the lowest frequency available on the standard AM radio broadcast band, i.e., 530 KHz. This radio signal consists of digital bits (ones and zeros) that are created by raising and lowering the transmitted power of the signal once every second. It takes about one minute to transmit a time code consisting of the one-second bits.
This time signal transmitted by radio station WWVB forms the standard for time in the United States and in other North American countries. It is utilized as the time standard by radio and television broadcast networks, by many entities on the internet, and wherever else accurate time is needed or desired. Many individuals also desire to have a clock that provides this highly accurate time.
Radio-controlled clocks satisfy this need by decoding the WWVB time signal and by synchronizing to the current time contained in the signal. RCCs also typically include appropriate hourly adjustments to compensate for differences of time between the different time zones, and for Daylight Savings Time (DST), such that an accurate time is displayed in each time zone where the RCC is used.
RCCs contain an antenna and a radio receiver that is constantly tuned to the 60 KHz frequency of radio station WWVB. Reception of the radio signal can be affected by many factors such as electromagnetic interference and the position of the antenna inside the RCC with respect to the WWVB transmitter at Fort Collins, Colo. As is typical with low frequency radio reception, it is known that this low frequency signal is usually stronger during the evening hours after the sun has set and before the morning sunrise.
Initial synchronization of the time displayed by the RCC to the time code within the WWVB signal typically takes at a couple of minutes, or longer. This is because it takes about one minute to transmit the time code in the signal. Rarely will the RCC be initially powered up to coincide with the beginning of a new time code transmission. Thus, the RCC needs to wait for the next complete time code. Signal reception conditions may also affect how quickly the RCC can decode and display the received time. When synchronization occurs, the time displayed will be accurate to within a fraction of a second. Thereafter, the RCC may seek to decode the WWVB signal only once to a few times per day to confirm that it is still on the correct time, or to correct the displayed time if necessary.
Further background information about NIST, radio station WWVB and RCCs can be found at the internet site http://www.boulder.nist.gov/timefreq/, including the related pages available at this site.
Turning now to the radio-controlled clock, prior art RCCs are known with both analog displays having hour and minute hands, and with digital displays. RCCs with digital displays are generally preferred over the analog displays because the display of the time is generally perceived as more precise. A popular component for a digital display is the liquid crystal display (LCD). However, LCDs are difficult to read in dim lighting conditions, such as are encountered during evening hours, without some form of backlighting. On the other hand, backlighting can be overly harsh to the eyes under the dim lighting conditions of the evening hours.
It is also known that RCCs may experience difficulty in receiving the time signal from radio station WWVB in the presence of electromagnetic radiation. For example, it is commonly recommended that an RCC not be located near a cathode ray tube (CRT) monitor of a computer system. However, those RCCs that operate from an alternating current power system commonly employ a power transformer inside the housing of the RCC. This power transformer also emits electromagnetic radiation, which can potentially interfere with normal operation of the RCC. This power transformer provides operating power to the low frequency receiver and to the other electronics housed within the RCC.
Mentioned above is the not insubstantial amount of time that is required for the RCC to decode the time signal, as well as the risk that the RCC may be unsuccessful due to poor signal conditions, interference, or the like. Prior art RCCs do not generally provide any means to inform the user about the status of the decoding efforts. For example, there is typically no indication whether a recent decoding effort has been successful, or unsuccessful. Similarly, there is no indication when the RCC is currently in the process of decoding a time signal.
There is also no indication of the strength of the received time signal in prior art RCCs. If the user is given knowledge that the received signal is weak, he/she can reposition or relocate the RCC to increase the strength of the received signal, thereby optimizing performance by receiving the best possible time signal.